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August 24, 2011

Remote Dwarf Planet Covered In Ice, Possibly Methane

by editor

Scientists at the California Institute of Technology (Caltech) have discovered that a dwarf planet at the edge of the solar system, dubbed 2007 OR10, is an icy world with about half its surface covered in water ice that once flowed from ancient, slush-producing volcanoes.

The red-tinged planet, nicknamed Snow White, may also be covered in a thin layer of methane, the last remnant of an atmosphere slowly being dissipated into space, astronomers said.

"You get to see this nice picture of what once was an active little world with water volcanoes and an atmosphere, and it's now just frozen, dead, with an atmosphere that's slowly slipping away," said lead author Mike Brown, professor of planetary astronomy at Caltech.

Snow White was discovered in 2007 as part of the PhD thesis of Brown's former graduate student, Meg Schwamb. The planet orbits the sun at the edge of the solar system, and is about half the size of Pluto, making it the fifth largest dwarf planet.

At the time, Brown had incorrectly guessed the planet was an icy body that had broken off from another dwarf planet named Haumea. This led him to nickname the planet “Snow White”. However, follow-up observations soon revealed that the planet is actually one of the reddest objects in the solar system.

A few other dwarf planets at the edge of the solar system are also red, and are part of a larger group of icy bodies known as Kuiper Belt Objects (KBOs). As far as the researchers could tell, Snow White, though relatively large, was unremarkable–just one out of more than 400 potential dwarf planets that are among hundreds of thousands of KBOs.

"With all of the dwarf planets that are this big, there's something interesting about them–they always tell us something," Brown said.

"This one frustrated us for years because we didn't know what it was telling us."

At that time, the Near Infrared Camera (NIRC) at the Keck Observatory was the best instrument astronomers had to study KBOs, said Brown. However, NIRC had just been retired, so no one could observe 2007 OR10 in detail.

"It kind of languished," Brown said.

Meanwhile, Adam Burgasser, a former graduate student of Brown's and now a professor at University of California, San Diego, was helping to design a new instrument called the Folded-port Infrared Echellette (FIRE).

Last fall, Brown and colleagues used this instrument with the Magellan Baade Telescope in Chile to take a closer look at 2007 OR10.

As expected, Snow White turned out to be red.

However, surprisingly, the spectrum revealed that the planet´s surface was also covered in water ice.

"That was a big shock," said Brown.

"Water ice is not red."

Ice is common in the outer solar system, but is almost always white.

There is, however, one other dwarf planet that's both red and covered with water ice: Quaoar, which Brown helped discover in 2002.

Slightly smaller than Snow White, Quaoar is still large enough to have had an atmosphere and a surface covered with volcanoes that spewed an icy slush, which then froze solid as it flowed over the surface.

But because Quaoar isn't as big as dwarf planets like Pluto or Eris, it could not hold onto volatile compounds like methane, carbon monoxide, or nitrogen as long.

A couple of billion years after Quaoar formed, it began to lose its atmosphere to space, leaving only some methane behind. Over time, exposure to the radiation from space turned that methane–which consists of a carbon atom bonded to four hydrogen atoms–into long hydrocarbon chains, which appear red.

Like the frost that covers a lawn on a cold morning, the irradiated methane sits on Quaoar's icy surface, giving it a reddish hue.

The spectrum of 2007 OR10 looks similar to Quaoar's, suggesting that what happened on Quaoar also happened on 2007 OR10.

"We're basically looking at the last gasp of Snow White. For four and a half billion years, Snow White has been sitting out there, slowly losing its atmosphere, and now there's just a little bit left."

Although Snow White's spectrum clearly shows the presence of water ice, the evidence for methane is not yet definitive, Brown said.

To find out for sure, astronomers will have to use a large, powerful telescope like the one at the Keck Observatory. If it turns out that Snow White does indeed have methane, it will join Quaoar as one of only two dwarf planets that straddle the border between the handful of objects large enough to hold onto volatile compounds, and the smaller bodies that make up the vast majority of KBOs.

Another task is to give the dwarf planet an official name, since "Snow White" was just a nickname he and his colleagues used, said Brown.

Furthermore, the moniker no longer makes sense for describing this very red object.

Prior to the discovery of water ice and the possibility of methane, "2007 OR10" might have sufficed for the astronomy community, since it didn't seem noteworthy enough to warrant an official name.

"We didn't know Snow White was interesting," Brown said.

"Now we know it's worth studying."

The research is published in the Astrophysical Journal Letters.

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Image Caption: An artist's conception of 2007 OR10, nicknamed Snow White. Astronomers suspect that its rosy color is due to the presence of irradiated methane. Credit: NASA